alt Hacker NewsBoron contamination of graphite also slowed the Manhattan Project:
" Fermi writes that "The results of this experiment was [sic] somewhat discouraging" presumably because of the absorption of neutrons by some unknown impurity. So, in December 1940 Fermi and Szilard met with Herbert G. MacPherson and V. C. Hamister at National Carbon to discuss the possible existence of impurities in graphite. During this conversation it became clear that minute quantities of boron impurities were the source of the problem. "
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Sounds like a good time travel story: the mission is to go back in time and plant a single uranium cube, to be found ~80 years hence, to convince people that the German bomb project was doomed so that– ahh, but let's not spoil it! :)
This is why the German's used heavy water instead of graphite as a neutron moderator, which made it much harder to build a reactor. Their early reactors consisted of concentric spheres of heavy water and uranium powder. This was an unfortunate combination, as uranium powder ignites on contact with water. Leaks caused explosions on two separate occasions [0].
In contrast, the first allied nuclear reactor was a literal pile of uranium and graphite on a squash court.
[0] The Bastard Brigade (great book on all of this)
[1] https://en.wikipedia.org/wiki/Chicago_Pile-1